I have a friend with a 65 valiant 225 slant six. Very stock car. Factory muffler is all rotted out. She would like a little more sound and is asking what muffler. I’m thinking ANY new muffler is going to be quiet compared to what she’s got and the tiny factory pipe is probably going to be replaced up to the manifold.

I’m thinking go with a little bigger pipe (2”?) and just run it all the way back, no muffler. Worst case she can insert a muffler afterwards if this sounds horrible.

Any thoughts?

Back in the day, a glass-pack muffler on those sounded OK and wasn't too loud, but definitely audible.
The low-rider guys would run dual exhaust ("twice-pipes"), which had an interesting harmonic.

A six with splits can rack down a V8 any day.

What would a straight piped (one pipe) six sound like compared to stock? I’m thinking not that much louder, cause stock sixes were pretty quiet.

Agree with split into duals with no muffler. Ran a 70 Swinger that way for twenty years. Sounds awesome.

if the whole exhaust needs to be replaced, this isn't a horrible price for mandrel bent exhaust:

https://www.ttiexhaust.com/3-Slant6.htm

if you want some more noise, I'd get it without the muffler, and replace the turbo muffler they use with one of these:
https://www.dynomax.com/mufflers/ul...ra-flotm-welded-offset-centered-699.html

Personally, I think no muffler on a slant 6 would sound terrible! When I was in High School in the 90's, I had a '74 Dart with a 225. I put a Purple Horny muffler on it and it sounded pretty cool.

I would have a muffler too. Pre emissions slant sixes sound pretty throaty for a six with a muffler and I think without one she'll get tired of the drone real quick.